Certain drugs used in the treatment of neoplastic and some non-neoplastic diseases produce human reproductive toxicity. Detailed studies of these patients before, during and after treatment may help elucidate the mechanisms of, and sites of reproductive toxicity. Parallel experimental studies in rodents and non-human primates will allow a more specific dissection of the mechanism of reproductive toxicity of these drugs as well as the sensitive reproductive processes. Cyclophosphamide, a commonly used alkylating agent has been demonstrated to destroy rodent primordial oocytes in a strain and species dependent fashion. Single intraperitoneal injections of cyclophosphamide do not destroy the growing or large oocytes/follicles in rodents. This appears in marked contrast to their action in humans. Azathioprine and 6-mercaptopurine do not destroy oocytes in weanling mice.